# Simple LED circuit.. HELP!

Discussion in 'Misc Electronics' started by [email protected], Nov 11, 2005.

1. ### Guest

I am having problems with this one, and it shouldnt be that hard...

I have a 9v battery, and I want an LED to be on above 6.5 volts and off
below 6.5. I don't want it to dim, I want it to be bright at 6.6 and
off at 6.4...

How do I do this?!

2. ### Don TaylorGuest

One way might be to include an op-amp and a few resistors to your
circuit.

circuit op amp hystresis
you can find some examples. The idea is that using a few resistors
you can make the op amp output switch from low to high at a fairly
specific voltage. You would probably use two resistors to create a
voltage divider to make your 6.5 volts. Then you would use two more
resistors to create the feedback needed to get the op amp to turn on
and off at that voltage. Then you would use the output of your op
amp to drive your LED and resistor.

3. ### CWattersGuest

For that level of accuracy you need something like a voltage comparator.

4. ### DaveMGuest

View this with a fixed pitch font such as Courier

+9V
|
+--------------------------------+
| |
| ------
| \ / LED
| \/
| -----
| |
| |
| \
| / 620
| \
| /
| |
| 10K |\ |
+--/\/\/\/\-----+-----| \ |
| | - | \ ______|
\ | | /
1.5 / +-------------| / LM339 or similar comparator
K \ | | + |/
/ | |
| | \
+-------+ / 27K
| \
-----/ /
/ ^ |
/ \ 4.7V |
--- 400 mw |
| |
| |
+---------------+
|
-----
---
-

Resistors are 1/4W 5%
If you want to adjust to exactly 6.5 V for switching, replace the 10K
resistor with a 7.5K fixed resistor in series with a 5K trim pot. Caveat...
this circuit does not provide hysteresis, so if the source is a bit noisy or
fluctuates around the switching point, the LED may flicker on/off. But
this certainly meets your original spec.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in

Never take a laxative and a sleeping pill at the same time!!

6. ### Guest

Alright, thanks for all the info guys! I will look into all your
solutions this morning. For the record, I am measuring my own supply
voltage.